Philips pm2521 - Page 173

1
REPAIR BY SIGNATURE ANALYSIS
1.
GENERAL
For repair by
signature analysis
a
testrom has been
developed.
This
testrom enables the user to test the digital part of the PM2521
by means of signature
analysis.
It also
enables troubleshooting at component level in
the circuit under test, by comparing
the actual
signature to the correct
one.
So the
trouble-shooter can trace back the faulty node.
In
total there are
8
tests. In the testrom 7 tests
have been implemented and one test is made
with
the
help
of
the
free run facility of the PM2521
.
2.
WHY USE A SIGNATURE ANALYSER ?
Thanks
to
the microcomputer in the digital part of the PM2521
,
the instrument
can be very small.
Due
to
this the bus structure is very complex. Data
flow
on the bus
becomes complex and cannot be analysed
with an oscilloscope and
voltmeter
any longer.
Therefore the need
to
test and service the digital part is acute.
The trouble-shooting involves the detection of
circuit failure and
the
localization of faults at the source.
Finding the fault source in
a
bus system, such as
the bus in the PM2521
,
with complex feedbacks can be
difficult.
But
opening the feedbacks paths and using the signature analyser makes
fault-finding easy.
3.
CHECKWITH
A SIGNATURE ANALYSER
The signature analyser checks the digital
part
of the PM2521 by detecting the bit stream at
various circuits
nodes, and displaying them
as
hexadecimal signatures.
To do this the PM2521 supplies the signature analyser with three gating signals, or in some cases
four:
START,
STOP, CLOCK
and QUALIFIER.
START signals the beginning of
a
measurement and prepares the signature analyser to recieve information
from the data probe. STOP closes this measurement. In cases of measuring signatures with RC delays,
a QUALIFIER has to be used to stop the incomming information for
a
short period of time. The CLOCK
is the system clock of the PM2521 under test to accept the data synchronously.
NOTE: The active edge of
each
of these
gating
signals
must
be selectable.
4.
TYPE
OF SIGNATURE ANALYSER TO BE USED.?
The signature analyser
to
be used
in
combination with
the PIV12521
should
have
the following capabilities:
—
Selectable edges of; START, STOP and CLOCK
-
AQUALIFIER
—
Signatures
in
the following
sequence:
DIGIT
DISPLAY DIGIT
DISPLAY
0 1000
8
1 1001 9
1010 A
:B
1
1011 C
1100 F
1101
H
B
^^B
1110 P
B ^^B
1111
U